Question

(M-10)(M+5)
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Answer 1

Let speed of stream be 's'

Let speed of boat in still water be 'b'

Speed of boat in still water = b = 5 km/h

Let time taken by the boat in going 40km downstream be t1 and time taken by the boat in going 40km upstream be t2.

• Time = distance/speed

• t1 = 40/(b+s)

• t2 = 40/(b-s)

He takes thrice as much as time going in 40km upstream as going 40km downstream.

• →t2 = 3t1

• →40/(5-s) = 3(40/(5+s))

• →1/(5-s) = 3[1/(5+s)]

• →5+s = 3(5-s)

• →5+s = 15-3s

• → s+3s = 15-5

• → 4s = 10

• → s = 10/4

• → s = 5/2